compact.scala

// Current Scala 2
trait Concat[A, B] {
  type Result 
  def apply(a: A, b: B): Result
}

object Concat {
  def apply[A, B](a: A, b: B)(implicit cab: Concat[A, B]): cab.Result = cab(a, b)

  implicit def hnil[B]: Concat[HNil, B] { type Result = B } = new Concat[HNil, B] {
    type Result = B
    def apply(a: HNil, b: B): Result = b
  }

  implicit def hcons[H, T, B](implicit ctb: Concat[T, B]): Concat[H :: T, B] { type Result = H :: ctb.Result } = new Concat[H :: T, B] {
    type Result = H :: ctb.Result
    def apply(a: H :: T, b: B): Result = a.hd :: ctb(a.tl, b)
}

// Possible compact syntax for the above ...

dep def concat[A, B](a: A, b: B): Result
  case (a: HNil, b: B): B => b
  case (a: H :: T, b: B): H :: ctb.Result => a.hd :: ctb
    where val ctb: concat(a.tl, b)